Recently an increasingly large number of electronic devices are equipped with so-called digital switches for setting up values and parameters in place of conventional means such as potentiometers and so on which may be considered as analog set up means. A digital switch has the advantage that an input may be made in terms of discrete numbers. This development may be compared to the transition from slide rules to electronic calculators.
Since a digital switch requires an electric part which may be consisting of a more or less conventional switch element and a mechanical part which activates the switch element. The ratchet mechanism is most commonly used for such a mechanical part. Therefore, a digital switch is a fairly complex device and has been often too expensive for practical applications in spite of a strong demand for the use of digital switches.
Conventionally, a digital switch has been comprised of a case, a indicator wheel having a ratchet gear integrally formed therewith, a push rod having a pawl at its inner end so that the pawl may cooperate with the ratchet gear so as to turn the ratchet gear, along with the indicator wheel, in a step-wise manner. The indicator wheel is further connected to a switch element having movable contact pieces and fixed contact pieces which may be arranged in a pattern so as to produce a desired combination of output signals at the output terminals depending on the rotational angle of the indicator wheel.
Recently, development in micro-electronics has created a need for extremely compact design of digital switches, but, because of the basic mechanical complexity, there have been no digital switches which are of sufficient compact design and both economical and reliable. Following are some of the problems which engineers have encountered in designing such a digital switch.
First, because the ratchet and pawl mechanism requires a large number of component parts and, therefore, assembly work tends to be cumbersome and it is difficult to assure sufficient mechanical strength to each component part.
Secondly, a digital switch is generally equipped with a stopper mechanism which determines the range of the values which may be set up and, in order to indicate it to the user that a limit has been reached, the stopper mechanism must have a sufficient mechanical strength to withstand the force the user may apply to the digital switch without knowing that the limit has been reached. Conventional, pins are pressure fitted into the holes in the ratchet gear and the rotational limit of the indicator wheel has been determined by the engagement of the pins with a projection on a fixed member. However, as the size of the ratchet gear is reduced, it becomes difficult to obtain enough areas on the ratchet gear for fitting metallic pins thereinto with sufficient mechanical strength.
Thirdly, as the size of the digital switch is reduced, so the size of the switch element must be reduced. As a result, a small misalignment in the pattern of contact pieces may cause errors in the operation of the digital switch. Conventionally, printed circuit boards have been widely used as contact pieces having various patterns, but such printed circuits may lack necessary durability on one hand, and may lack sufficient dimensional precision on the other hand. Additionally, manufacturing a printed circuit requires special materials and special facilities, resulting in a relatively high cost for manufacture.
Alternatively, an electro-conductive pattern has been made by forming a V-groove corresponding to the pattern in the side surface of the circular base plate at the time of molding the same and pressure-fitting an electro-conductive member made of a printed circuit into the V-groove. However, because the electro-conductive pattern comprises a continuous contact surface having a circumferential portion, a radial portion and another concentric circumferential portion and a plurality of isolated contact surfaces, manual labor required for fitting the electro-conductive members into the V-grooves of the indicator wheel has been so substantial that it has been a major factor for the high cost of such a switch and increasingly compact design of switches tends to reduce the efficiency of such work.